1. Field of the Invention
The present invention relates generally to, and particularly to electrical wiring devices, and particularly to protective electrical wiring device.
2. Technical Background
AC power is provided to a house, building or other such facilities by coupling one or more breaker panels to an electrical distribution system, or another such source of AC power. The breaker panel distributes AC power to one or more branch electric circuits installed in the structure. The electric circuits typically include one or more receptacle outlets and may further transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. The receptacle outlets provide power to user-accessible loads that include a power cord and plug, with the plug being insertable into the receptacle outlet. Because certain types of faults have been known to occur in electrical wiring systems, each electric circuit typically employs one or more electric circuit protection devices. Electric circuit protective devices have been disposed within the breaker panel, receptacle outlets, plugs and the like.
Both receptacle wiring devices and electric circuit protective wiring devices in general, are disposed in an electrically non-conductive housing. The housing includes electrical terminals that are electrically insulated from each other. Line terminals couple the wiring device to conductors that provides electrical power from the electrical distribution system. Load terminals are coupled to wiring that directs AC power to one or more electrical loads. Those of ordinary skill in the pertinent art will understand that the term “load” refers to an appliance, a switch, or some other electrically powered device. Load terminals may also be referred to as “feed-through” terminals because the wires connected to these terminals may be coupled to a daisy-chained configuration of receptacles or switches. The load may ultimately be connected at the far end of the branch circuit.
Referring back to the device housing, the load terminals may be connected to an electrically conductive path that is also connected to a set of receptacle contacts. The receptacle contacts are in communication with receptacle openings disposed on the face of the housing. This arrangement allows a user to insert an appliance plug into the receptacle opening to thereby energize the device. A circuit interrupter disposed between the line terminals and the load terminals provides power to the load terminals under normal conditions, but breaks electrical connectivity when the protective device detects a fault condition in the load circuit.
There are several types of electric circuit protection devices readily available. Examples of such devices include ground fault circuit interrupters (GFCIs), ground-fault equipment protectors (GFEPs), and arc fault circuit interrupters (AFCIs). This list includes representative examples and is not meant to be exhaustive. Some devices include both GFCIs and AFCIs. As their names suggest, arc fault circuit interrupters (AFCIs), ground-fault equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs) perform different functions.
An arc fault typically manifests itself as a high frequency current signal. Accordingly, an AFCI may be configured to detect various high frequency signals and de-energize the electrical circuit in response thereto. A ground fault occurs when a current carrying (hot) conductor creates an unintended current path to ground. A differential current is created between the hot/neutral conductors because some of the current flowing in the circuit is diverted into the unintended current path. The unintended current path represents an electrical shock hazard. Ground faults, as well as arc faults, may also result in fire.
A “grounded neutral” is another type of ground fault. This type of fault may occur when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded. While this condition does not represent an immediate shock hazard, it may lead to serious hazard. As noted above, a GFCI will trip under normal conditions when the differential current is greater than or equal to approximately 6 mA. However, when the load neutral conductor is grounded the GFCI becomes de-sensitized because some of the return path current is diverted to ground. When this happens, it may take up to 30 mA of differential current before the GFCI trips. Therefore, if a double-fault condition occurs, i.e., if the user comes into contact with a hot conductor (the first fault) when simultaneously contacting a neutral conductor that has been grounded on the load side (the second fault), the user may experience serious injury or death.
Like all electrical devices, protective devices such as GFCIs, AFCIs, and other such devices have a limited life expectancy. When a protective device has reached end of life, the user may not be protected from the fault condition. End of life failure modes include device circuitry failure, circuit interrupter failure, relay solenoid failure, and/or solenoid switching device failure. Note that switching devices include thyristors such as the silicon controlled rectifier (SCR). For the sake of brevity, switching devices are hereinafter referred to as “SCRs.”
Because end of life failure modes may result in the user being unprotected from the faults referred to above, test buttons have been incorporated into protective devices to provide the user with a means for testing the effectiveness of the device. One drawback to this approach lies in the fact that if the user fails to use the test button, the user will not know if the device is functional. Even if the test is performed, the test results may be ignored by the user for various reasons.
One of the failure modes listed above relates to the GFCI becoming inoperative when the SCR reaches end of life (shorts out). Some GFCI devices will continue to deliver power to the load circuit even though the device is non-protective. However, this approach leaves the user unprotected in the event of a fault condition.
In another approach that has been considered, a GFCI may be configured to trip the circuit interrupter in the event that the SCR shorts out. When the device trips out, the user will attempt to reset the device only to find that it immediately trips again. Consequently, the GFCI prevents power from being delivered to the load. However, this approach also has several drawbacks associated with it. It may be some time before the user realizes that the GFCI has tripped—the user only discovers that power is not available at the exact moment that power is needed for the user's application. In response, the user attempts to reset only to discover that the device trips out again in the manner described above. Subsequently, the user initiates a search for a receptacle that is still functional. The receptacle may be disposed in a different room requiring the user to employ an extension cord.
The trouble-shooting process may be further complicated by the GFCI's inability to differentiate between an internal fault (i.e., shorted SCR) and an external fault condition (i.e., a ground fault in the protected circuit). Accordingly, the GFCI may continue to trip in response to both conditions. This forces the user to guess. If the user assumes that power denial is due to a malfunctioning GFCI, the user will replace the GFCI with a new one only to discover that the GFCI was denying power in response to an external fault condition. The user then examines the branch circuit to determine the source of the fault condition. Trouble shooting by “trial and error” is costly, time-consuming, and a source of aggravation to the user.
What is needed is a protective device that denies power to the protected circuit when the device is non-protective. What is needed is a protective device that provides early indication to the user of power denial to the load circuit. What is needed is a protective device that helps trouble shoot the cause of a power denial to the load circuit.